The Best Flower Seeds to Start Indoors & How to Do It

The Best Flower Seeds to Start Indoors (Top 5 Picks)

Cosmos: Fast, forgiving, and brilliant for cutting, the cosmos flowers strongly right up to the first frosts. Sow in March or April indoors, and you’ll see germination happen in a week. It’s important not to start sowing these too early, as they grow fast and turn leggy waiting for warmth.

Zinnia: Zinnias need heat and a long run after sowing, so start indoors in April. We advise that you sow in individual modules rather than open trays. They hate root disturbance, so sow separately for flowers in July through to October. Available in eye-catching single colours, gorgeous bicolours, and tricolours, there are zinnia seeds for every garden style and planting scheme.

Lobelia: These seeds are fairly small, so when planting, mix them with a pinch of dry silver sand before sowing so you can see where they’ve gone. As they need light to germinate, sow on the surface of your seed tray in late February or March. Germination indoors can take three weeks, but your lobelia seeds will grow to deliver compact, deep-blue cascading blooms for borders, hanging baskets, and containers.

Petunia: Petunias need more time than almost anything in the bedding world, so start sowing your petunia seeds in February. Like lobelia, they need light to germinate, so press them onto the surface uncovered. Petunias are fast-growing and will germinate quickly, giving blooms from early summer to the first frost.

Antirrhinum: Also known as snapdragons, these can be sown indoors from February. Make sure you pinch out the tips when plants are a few inches tall. It may feel wrong, but it makes for a far bushier, stronger plant, and will reward you with tall, elegant flower spikes in beds and borders.

How to Start Flower Seeds Indoors: A Step-by-Step Guide

Choose your container: Seed trays suit small seeds like lobelia and petunia. Module trays are better for larger seeds, and having one per cell means less root disruption when it’s time to plant out.

Use the right compost: We recommend using seed and cutting compost only, and not general-purpose. Regular compost can scorch seedlings before they’ve properly emerged. Make sure you fill your seed tray or modules to just below the rim and firm lightly.

Sow at the right depth: Many flower seeds need light to germinate and should be planted on the surface, uncovered. For larger seeds, like cosmos or zinnia, make sure you cover with a thin layer of vermiculite rather than compost. It’s sterile, drains well, and lets light through.

Water from below: Set the tray in a shallow dish of water for 20–30 minutes, then drain. Watering from above can shift tiny seeds and can bury light-dependent ones.

Get the temperature right: Most half-hardy annuals want 18–24°C temperatures at the soil level. A heated propagator nails this, so make sure you check the seed packet instructions and propagator temperature after sowing.

Don’t neglect seedlings once they’re up:  The most common mistake is leaving the lid on too long. Remove it gradually once seedling growth appears. Wet, still air causes damping-off and rot that collapse seedlings at the base. Make sure you keep compost moist, not sodden.